The controlled generation or destruction of vortices plays an important part in many flow-related applications. Since vortices contain rotational energy and can therefore both absorb and discharge energy, they are used for the controlled transmission of energy from a flow-around profile to the flow medium or into a boundary layer or for the exchange of energy, in particular including heat, or of substances between different regions of a flow.
For the efficient transport of a flow medium, for example, systems are used consisting of stationary vortex generators which transmit energy to the flow medium as a result of the generation of vortices. In this case, it is desirable to keep the pressure loss which the flow medium experiences along the system consisting of vortex generators as low as possible. In particular, the mean mass flow with respect to the pressure loss is to be as high as possible.
Owing to the use of vortex generators in the flow duct, velocity fluctuations can also be imparted to a flow medium. Such velocity fluctuations, which are generated, for example, by means of shockwaves occurring at vortex generators, considerably increase both heat exchange and mass transfer in the flow medium transversely to the flow direction. By virtue of the targeted mounting of vortex generators in the flow duct, an intensified cooling action of the flow on components subjected to especially high thermal load can thus be achieved. In this case, the vortex generators should be positioned and dimensioned in such a way that the heat transmission coefficient in relation to the pressure loss which the flow medium experiences along the system consisting of vortex generators is as high as possible. Thus, for example due to the use of a system consisting of vortex generators in a gas turbine, cooling air can be saved both in the region of the combustion chamber and in the region of the turbine blades, and consequently, with at the same time the efficiency of the gas turbine being increased, its NOx emissions can be lowered.
The intensified mass transfer transversely to the flow direction in a flow medium with velocity fluctuations may be utilized for the intensified mixing of the flow medium. For example, owing to the especially thorough mixing of fuel gas and air in a gas turbine plant, a complete combustion of the fuel gas can be achieved and therefore its NOx emissions can be lowered.
The diversity of technical applications of vortex generators or vortex destroyers means that there is great interest in the theoretical calculation of the occurrence and development of vortices. They are necessary, in particular, in order to configure the shape and positioning of vortex generators and vortex destroyers optimally in terms of their action on a flow medium. The calculation of turbulent flows is conventionally carried out either explicitly by solving the Navier-Stokes equation of the problem, although this is a procedure which is too complex and involves too high an outlay, above all in the case of three-dimensional applications, or else via corresponding models of the classic hydrofoil theory.
Within the framework of the classic hydrofoil theory, admittedly, turbulent flows at components which are rigid, that is to say with a passive flow around and not accelerated, can be described. However, with components which are active, that is to say moved in an accelerated manner, the classic hydrofoil theory falls down. To be precise, it presupposes the smooth flow-off of the flow medium at the trailing edge of the flow-around profile, what is known as the Kutta condition, and also a finite onflow velocity and a quasistatic linear treatment. A breakaway of the flow from the flow-around profile and a rolling-up of vortices which occur, such as takes place in the case of non-stationary flow processes on profiles moved in an accelerated manner, cannot be treated within the framework of the theory. Where technical applications are concerned, therefore, vortex generators with a passive flow around are normally used, which can be described by means of classic aerodynamics. However, vortex generators with a passive flow around have comparatively high dynamic resistances. If they are placed in a flow duct, this therefore results in an undesirably high pressure drop in the flow medium.